One digital-book store, Wowio Inc., is making inroads selling ads in the e-books users download from its site onto, say, laptops or e-readers like Apple's iPad and Amazon's Kindle. Some Wowio e-books have three pages with promotions: an introduction and a closing page each with an ad, plus another full-page ad. The company also is experimenting with techniques to insert ads between chapters and to target ads using profile information that users submit to its website, says Wowio Chief Executive Brian Altounian.

The movie site Fandango is among the Los Angeles company's clients. Fandango is giving Jonathan Swift's "Gulliver's Travels," with three pages of Fandango promotions, to people who buy tickets on the site to the Jack Black movie "Gulliver's Travels," which opens on Christmas.

"It is not the kind of thing where you are reading and a video pops up on the screen," Mr. Altounian says. "If advertising gives access to content that is free or heavily subsidized, then most readers will accept it."

Wowio charges advertisers between $1 and $3 for each book downloaded and shares revenue with the publisher. The publisher determines how much of those ad dollars trickle down to the author.

Wowio has deals with such companies as RosettaBooks LLC, whose titles include Kurt Vonnegut's "Slaughterhouse Five," and Arcturus Publishing Ltd., which publishes a broad range of books, including how-to titles and histories.

Companies with other business models also are looking for ways to infuse digital books with advertising. Scribd Inc., a social-networking and self-publishing site focused on reading, is trying ads that are relevant to users based on what they are reading or their stated interests. ScrollMotion Inc., which contracts with publishers to produce electronic versions of their books, is at the early stages of seeking ways to put ads in books.

The market for digital books is $966 million this year and is expected to grow quickly in the coming year, according to Forrester Research Inc.

But ads in e-books are likely to be a tough sell. While it was typical for books a century ago to be published in serial form with ads, in-book advertising now is atypical. For starters, most books sell only a few hundred thousand copies, not enough to interest most advertisers. And many author contracts say the writer has to approve any ads.

The lifespan of books, meanwhile, is such that an ad that appears when it first was published could be irrelevant years later. But digital books can address that problem by inserting ads that are appropriate for when a person accesses the book and targeted to the reader's interests. Some companies also are working to sell space across a number of books to entice marketers to purchase ads.

Regardless, ads will have to overcome the annoyance factor. Imagine an ad for a sports drink that says "Is your day feeling like the worst of times?" that appears in "A Tale of Two Cities" next to the line "It was the best of times, it was the worst of times," or ads for condoms interspersed through "The Scarlet Letter," says Forrester analyst James McQuivey.

"It would be an unpleasant distraction in the middle of a narrative," says literary agent Ann Rittenberg. "This is going to be a lot more complicated than putting an ad at the back of a paperback."

Stuart Applebaum, a spokesman for Bertelsmann AG's Random House Inc., says ads often appeared in the back of mass-market paperbacks in the 1950s and early '60s. But ads were never a big revenue source and the practice was abandoned for a variety of reasons, including author objections.

Inserting ads in Random House e-books won't happen without author approval, Mr. Applebaum says.

"It's a nonstarter here without their assent, regardless of format," he says. "However, if our authors were ever to be agreeable to it, it might have some traction."

Write to Emily Steel at emily.steel@wsj.com